The present invention relates to a process for producing sucrose from sugar beets.
The conventional beet sugar manufacturing process involves cleaning the beets, slicing them into cossettes, extracting juice from the cossettes by diffusion, purifying the juice by liming and carbonation, concentrating the juice by multiple effect evaporation, multi-stage boiling of concentrated juice in pans, separation, washing, and drying the sugar.
Juice extraction in the conventional process is done by allowing the sugar to diffuse through the natural cell walls of beets. The cell walls allow sugars and other low molecular weight compounds to pass through but prevent the passage of high molecular weight compounds. This selective diffusion process has two advantages. Retaining the high molecular weight compounds helps produce a high purity juice. It also reduces filtration difficulties that are caused by polysaccharides and proteins that comprise the high molecular weight compounds.
Purification of beet juice in the conventional process is based on lime treatment. Lime serves many purposes in the juice purification process. It neutralizes the acidity of the juice and precipitates calcium salts of several organic and inorganic acids. The precipitate absorbs other impurities. The lime precipitate produces a porous mass, which facilitates subsequent filtration of juice.
The conventional diffusion process for juice extraction from beets has two disadvantages. It has a long retention time, which encourages microbial growth, resulting in sugar loss and formation of undesirable compounds. Also the diffusion process has limited extraction capability, leaving about 2-5% of the original sugar in the pulp. This pulp is pressed and the press juice is introduced back into the diffuser. A significant portion of the high molecular weight compounds retained by the cell walls in the diffusion process is released in pressing to be mixed with the diffusion juice. This partially negates the advantages of the selective diffusion process.
The conventional liming process uses large quantities of lime, amounting to about 2.5% of the total weight of beets processed. Beet sugar plants operate lime kilns and transport limestone over long distances for this purpose. The effluent from the liming-carbonation process, consisting of used lime and separated impurities, is disposed as waste. Production of lime and disposal of liming effluent are costly operations. Disposal of liming effluent is becoming increasingly difficult and expensive in many communities.
Conventional dead-end filtration is incapable of separating sucrose from macromolecular impurities in beet juice. Several methods of using microfiltration and ultrafiltration for purification of juice with reduced lime use have been reported, but these methods generally involve inserting microfiltration or ultrafiltration membranes into the conventional beet process at one or more points.
There is a long-standing need for improved processes for obtaining sugar from beets that avoid or at least minimize one or more of the problems existing in the previously used processes.
The present invention relates to a process for producing sugar from beets, comprising the steps of (a) macerating beets or pieces thereof; (b) mechanically separating juice from the macerated beets; and (c) membrane filtering the separated juice, producing a retentate and a permeate. The present invention makes use of mechanical means, such as vacuum filtration, for separating juice from macerated beets, as opposed to the simple diffusion process that is used in prior beet processing technology to obtain juice from cossettes.
In certain preferred embodiments of the process, where beets are cut into pieces and subsequently macerated, and the maceration is done in an attrition mill. It is also preferred that vacuum extraction of juice is done on a moving porous filtration belt with countercurrent flow of macerated beets and water, most preferably at a temperature of at least about 80xc2x0 C. The pH of the vacuum extracted juice preferably is adjusted to at least about 7 by addition of sodium hydroxide.
In one preferred embodiment of the process, the extracted juice is contacted with an agent selected from the group consisting of sulfur dioxide, sulfate salts, sulfite salts, bisulfite salts, and mixtures thereof, in an amount sufficient to adjust the pH of the extracted juice to no greater than about 8.
The membrane filtration can suitably be done with an ultrafiltration membrane, a nanofiltration membrane, or other types of membranes described herein. In one preferred embodiment, the membrane filtration is cross-flow ultrafiltration, and is done at least about 80xc2x0 C, and the pH of the permeate is at least about 7.
One preferred option in the process is to subject the retentate from the membrane filtration to diafiltration, in order to recover residual sugar in the retentate, thereby producing a diafiltration filtrate (also referred to herein as diafiltrate). This diafiltrate preferably is combined with the membrane filtration permeate for further processing.
Another preferred option in the process is concentration of the permeate from the membrane filtration by reverse osmosis, thereby producing a concentrated solution. This concentrated solution is evaporated and sucrose is crystallized therefrom.
Preferably in the process of the present invention no lime and no carbon dioxide are contacted with the juice or the permeate.
One specific preferred embodiment of the process comprises the steps of: (a) cutting sugar beets into pieces; (b) macerating the beet pieces; (c) mechanically extracting juice from the macerated beets; (d) sulfitation of the extracted juice; (e) pH adjustment of the extracted juice to at least about 7; (f) membrane filtering the extracted juice, producing a retentate and a permeate; (g) subjecting the retentate to diafiltration, thereby producing a diafiltration filtrate that is enriched in sugar compared to the retentate; (h) combining the diafiltration filtrate and the permeate from the membrane filtration, thereby producing a combined juice; (i) concentrating the combined juice by reverse osmosis, thereby producing a concentrated solution; and (j) evaporating the concentrated solution and crystallizing sucrose therefrom.
The process of the present invention has many advantages over the conventional process using diffusion, liming and carbonation. For instance, this process has a lower retention time, which reduces the extent of microbial destruction of sucrose. The fineness of the macerated beets reduces the percentage of sucrose retained in the pulp to below about 0.5% compared to as high as 0.75% in the conventional process. Higher extraction due to maceration and reduction in inversion due to reduced retention time increase the total sugar recovery by about 1 to 2% of the weight of beets processed.
This method of purification produces a beet juice of lower color than the traditional diffusion and carbonation process. Less color in the juice allows for less washing of the final crystalline product. Membrane filtration removes macromolecules in the beet juice, producing syrups of lower viscosity. Lower viscosity syrups crystallize faster and purge easier from the sucrose crystal surface. Low color, low viscosity syrup, reduces recycle during the crystallization process, resulting in better sugar recovery.
The process eliminates the lime kiln, lime quarries and all associated equipment, processes, products, by-products and waste products. Sodium hydroxide for neutralization of juice costs about 50% less than the lime that it replaces. Sodium hydroxide is easier to handle, cleaner and less abrasive on equipment than lime.
Also, the present invention results in a drastic reduction of waste products that cause environmental pollution. The conventional process produces a filter cake that comprises products of the liming process and impurities removed from the juice. This cake is disposed into ponds or landfills. The proposed process completely eliminates the need for disposal of such materials. Invert sugars end up with the molasses which is a salable byproduct and not in the effluent. The present invention also allows elimination of the carbonation process, which is a major source of atmospheric pollution in beet sugar plants.